The importance of providing the best possible protection against overheating at any point along an electrical resistance type baseboard heater due to inadvertent blocking of convection air flow by drapes, furniture or other objects is well known and understood. Protective devices for this purpose have conventionally consisted of an expansible chamber actuated cut-off switch with an elongated sensing bulb or capillary tube containing a thermally expansible fluid connected to the expansible chamber and extending substantially the length of the heater.
Uniformity, reliability and quick response to overheating are of course of primary importance and a quicker response time of the cut-off switch permits the safe operation of the heating element at higher output. The positioning of the sensing capillary tube relative to the heating element and in the convection air stream is critical if reliable operation and quick response is to be achieved.
We have found by extending a sensing capillary tube containing a relatively low boiling point fluid along and in fixed predetermined spaced relationship with a conventional elongated electrical resistance heating element equipped with longitudinally spaced, radially extending fins, as by attaching the capillary to peripheral portions of at least some of the fins, that sufficient cooling by a free flowing convection air stream to preclude cut-off and a rapid cut-off in response to blockage of air flow may be achieved.